1. Field of the Invention
The present invention relates generally to lithographic apparatus and device manufacturing methods using lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A pellicle is a thin, transparent film that is attached by a frame to the chrome side of a mask or reticle used in lithography. The pellicle and its frame keep the space near the chrome of the mask free of dust particles. In this way, any dust particles near the mask are kept away from the best object plane and will thus be imaged out of focus and will not cause defects. However, the standard pellicle material, a polymer film, can not be used when 157 nm radiation is used as the exposure radiation. The polymer film is not transparent to this wavelength and the exposure radiation would deteriorate the optical properties of the pellicle.
Therefore, a new pellicle material that is suitable for 157 nm radiation has been sought. The best solution found so far is to use fluorine-doped quartz. This is an inorganic material. In order to use it as a pellicle, the pellicle thickness has to be substantially increased, e.g., to 800 μm. Because the pellicle material has a different refractive index than the surrounding gas, such a thick pellicle causes imaging effect which cannot be neglected.
The main effect is an observed object plane shift. From behind the pellicle the object appears to be closer than it actually is. With an 800 μm thick pellicle that has a refractive index of 1.5, the observed object plane shift is about 280 μm. Because the object appears to have moved towards the projection system, this can be corrected by moving the mask stage away from the projection lens by about 280 μm. However, any deviations from planarity or in thickness of the pellicle will introduce other aberrations which will have undesirable effects on imaging.